I’d recommend Le Guin’s ‘The Dispossessed’ over ‘Left Hand of Darkness’ (but that’s also good, and given your stated interests I expect you’ve already read it). My favourite PKD is ‘A Scanner Darkly’, but ‘Man in the High Castle’ is good too. Pratchett’s first Discworld book was definitely not the best; although I ran out of steam with the series years ago, I still remember ‘Mort’ and ‘Small Gods’ fondly. What else… Snow Crash is great, and a good place to start on Stephenson’s work. I preferred The Martian Chronicles to Fahrenheit 451. Dune is a good read, the sequel isn’t, and so I never got beyond that second instalment. Timescape is one of my favourite SF novels, so it’s pleasing to see it make the list – one of the best depictions of real scientists in SF.
Others? I enjoyed The Space Merchants by Pohl and Kornbluth, which used to make these lists regularly; Robert Silverberg had a great run of novels in the 1970s; Heinlein’s later ‘grown up’ novels are tedious libertarian stuff, but his earlier ones are fun – ‘Have Space Suit, Will Travel’ was one of my childhood faves. My favourite shameless space opera would probably be Harry Harrison’s Stainless Steel Rat series.
I could go on all night…
[And now I’ve checked your numbers, and see that you have in fact not only read but taught two of the above.]